1. Field of the Invention
The present invention relates to a focusing control device that performs focusing by using image signals obtained by an imaging element performing photoelectric conversion of subject images focused by a photographic optical system.
2. Description of the Related Art
Conventionally, various techniques of autofocus (AF) during continuous shooting of a camera have been proposed. For example, Japanese Patent Application Laid-Open No. 2002-122773 discloses a technique as described below.
A scan area of a focusing lens may be determined that areas of the same width are distributed on both sides of the current in-focus position. Alternatively, according to a view point described below, a scan area may be determined such that different widths of area are distributed on both sides taking the current in-focus position as a reference.
During a continuous shooting, images are continuously taken at a short time interval and the moving direction of a subject is usually the same among successive shootings. So that, based on the moving direction of the subject during previous shootings, in other words, based on the driving direction of a focusing lens (moving direction of the focusing lens) during previous shootings, the in-focus position for the next shooting can be predicted by some degree.
FIGS. 13A to 13C illustrate in-focus positions at successive three shootings. In the shooting illustrated in FIG. 13C, the in-focus position in the previous shooting (FIG. 13B) is taken as a reference point and distributed scan areas (SC1 and SC2) in opposite sides of the reference point are set differently based on the moving directions of the in-focus positions in previous shooting (FIG. 13B) and the shooting before the previous shooting (FIG. 13A). The distributed scan area on the side to which the moving direction of a subject in previous shootings is directed is relatively set wide.
As in this manner, by varying distributed scan areas based on the moving direction of the in-focus position in previous shootings, the focusing lens can be driven efficiently and the speed of focusing process can be raised.
Japanese Patent Application Laid-Open No. 2013-57960 discloses a technique of cutting down on unnecessary lens drive by stopping the focusing lens when it is determined that a subject is not moving toward or away from a camera and keeping the lens stopped until the subject moves, changing the distance.
However, the technique disclosed in Japanese Patent Application Laid-Open No. 2002-122773 only scales distributed scan areas and does not change the reference point, and thus cannot be applied for a scene where the shifting speed of an image plane gradually increases. For a typical subject that approaches at a constant speed, the shifting speed of an image plane increases at an accelerated rate. Conventional techniques thus may have difficulty in focusing a typical moving subject.
Moreover, since the processing is unchanged regardless of the result of scanning, when focus cannot be set on a subject after scanning, inappropriately distributed scan areas may cause continuation of false focusing even for a still subject.
In the technique disclosed in Japanese Patent Application Laid-Open No. 2013-57960, it often happens that reliability of prediction for a low contrast subject or under a low-light intensity condition is determined to be low and the future position of a moving subject cannot be predicted, which may cause difficulty in continuously focusing the moving subject.